1. Field of the Invention
The present invention relates to a light-shielding film for optical element that is applied to an optical apparatus, such as a camera, binoculars, a microscope, or a semiconductor exposure apparatus, and relates to an optical element.
2. Description of the Related Art
A light-shielding film for optical element is a coating film formed on a surface of, for example, glass or plastic. The optical element may be a lens, a prism, or another optical glass element, but, in this specification, the light-shielding film will be described using a lens as an example.
As shown in FIG. 1, the light-shielding film 1 for optical element is formed on an appropriate outer portion of a lens 2 serving as the optical element. Some of incident light, such as incident light 3, passes through the lens 2 as transmitted light 4. On the other hand, oblique incident light, such as incident light 5, strikes the light-shielding film 1. If the light-shielding film 1 is not provided, the light that struck the outer portion of the lens 2 is inner-surface-reflected and exits to the outside of the lens 2 as image-unrelated inner-surface-reflected light 6, which causes flare, ghost, etc. to deteriorate the image quality. Occurrence of the inner-surface-reflected light 6 due to the oblique incident light 5 can be reduced by providing the light-shielding film 1, which can prevent flare and ghost to inhibit disadvantageous effects on an image.
FIG. 2 is a schematic view illustrating how inner-surface-reflected light travels. As shown in FIG. 2, incident light 3 travels in the lens 2 and becomes first reflected light 8 at the interface 21 with the light-shielding film 1. The transmitted light 9 traveled in the light-shielding film 1 becomes second reflected light 10 at the interface 22 between the light-shielding film 1 and air. Therefore, in the inner-surface reflection, the first reflection light 8 and the second reflection light 10 are involved.
Recently, along with a reduction in lens size and an improvement in performance, the designed clearance between a lens and a lens barrel has been reduced. Accordingly, if a light-shielding film for optical element has a thickness that is equivalent to those of existing films, since the clearance is small, the lens may not be incorporated into a lens barrel. Therefore, in order to set a lens provided with a light-shielding film into a narrow clearance, the light-shielding film for optical element needs to be reduced in the thickness. In addition, a thinner light-shielding film can decrease the stress, resulting in a reduction in deformation of the lens.
Japanese Patent Publication No. 47-32419 describes an example of the light-shielding film that absorbs light with coal tar, carbon black, and dye while improving the refractive index with the coal tar. Japanese Patent Laid-Open No. 2007-183444 describes an example of the light-shielding film that absorbs light with coal tar and dye while improving the refractive index with the coal tar. Japanese Patent Laid-Open No. 07-82510 describes in this coating film, the content of the inorganic black particle is 10 to 60 parts by weight, because a content not larger than 10 parts by weight cannot sufficiently increase the refractive index of the light-shielding film, resulting in a large difference between the refractive indices of the light-shielding film and an optical element not to inhibit inner-surface reflection.
In order to inhibit the above-described inner-surface reflection, it is necessary to decrease the first reflected light 8 and the second reflected light 10. In order to decrease the reflection at the first interface, it is effective to decrease the difference between the refractive indices of the light-shielding film 1 and the lens 2. That is, it is necessary that the light-shielding film for optical element have a refractive index near that of the lens. In order to decrease the reflection at the second interface, it is necessary to make the light-shielding film 1 sufficiently black for absorbing the transmitted light 9 that transmitted to the light-shielding film 1. That is, the light-shielding film 1 needs to have a degree of blackness that can sufficiently absorb the transmitted light entered inside the light-shielding film.
However, an increase in absorption by increasing the degree of blackness of the light-shielding film 1 causes a problem in that the first reflected light 8 increases. The absorption of the light-shielding film 1 can be also increased by increasing the thickness of the light-shielding film 1, but an increase in the thickness inhibits the above-described reduction in lens size and improvement in performance. The light-shielding film for optical element described in Japanese Patent Publication No. 47-32419 contains 15 wt % or more and 36 wt % or less of carbon black and 15 wt % or more and 36 wt % or less of a dye, which sufficiently increases the refractive index and reduces the difference between the refractive indices of the light-shielding film and the lens. However, since the absorption of the light-shielding film is high, the reflection at the interface between the lens and the light-shielding film cannot be sufficiently inhibited.
In the light-shielding film for optical element described in Japanese Patent Laid-Open No. 2007-183444, the refractive index is increased by coal tar to reduce the difference between the refractive indices of the light-shielding film and the lens. In addition, the absorption is small due to the low concentration of the dye, which can reduce the reflection at the interface between the lens and the light-shielding film. However, since the absorption is small, in order to inhibit the reflection at the interface between the light-shielding film and air, the thickness of the film must be increased.
In the light-shielding film for optical element described in Japanese Patent Laid-Open No. 07-82510, the inorganic black particle content is controlled in the range of 10 to 60 parts by weight in order to sufficiently increase the refractive index of the light-shielding film. This light-shielding film can have a high refractive index, but the degree of absorption of the light-shielding film is low. Therefore, the first reflected light 8, which is light reflected at the interface between the light-shielding film and air, cannot be sufficiently inhibited.